As the same with most power supply products, the development trends of DC/DC converter is towards high efficiency, high power density, high reliability and, low cost. For a conventional ZVS DC/DC converter with the diode rectification, the conduction loss of the diode rectifier is normally about 30%–40% of the total loss of the converter. Obviously, in order to further improve the efficiency of the DC/DC converter, it is a good idea to reduce the conduction loss of the rectifier. Synchronous rectifier, which has the lower conduction loss, may be used in DC/DC converter. Although the employment of the synchronous rectifier can significantly reduce the conduction loss, the driver issue of the synchronous rectifier has to be considered.
From the point of view of the synchronous rectifier, the ideal operation mode of the driver is: turning on the synchronous rectifier as soon as the body diode conducts, and turning off the synchronous rectifier as soon as the current flows through the synchronous MOSFET crosses to zero. The advantage of the aforementioned mode is that the conduction time of the body diode is nearly zero to avoid the additional conduction loss and the reverse recovery relevant loss.
Obviously, the control of the synchronous rectifier contains two key points: turning on and turning off. If the synchronous rectifier is turned on before the voltage VDS decreased to zero and body diode conducted, it will result in the parasitic capacitor discharging loss: P=0.5CossVDS2fs. If the synchronous rectifier is turned on after the diode conducted by time τ, it will result in the additional conduction loss: P=Id(VD−VMOS) τfs. If the synchronous rectifier is turned off before the current of MOSFET Id decreased to zero, it means the body diode conducts current just before it was turned off. It will cause not only the additional conduction loss, but also severe reverse recovery losses. Normally, the higher voltage rating of the MOSFET is, the poorer reverse recovery characteristics of the body diode is. If the synchronous rectifier is turned off after the MOSFET current Id crossed zero, the direction of the MOSFET current Id will be changed. As a result, after turning off the MOSFET, the severe voltage overshoot occurs across the drain to source terminal of the MOSFET so that the MOSFET suffers the risk of voltage breakdown.
Typically, there are two types of the synchronous rectifier driving modes: current driving mode and voltage driving mode. The principle of the current driving mode is very simple. To sense the current flowing through the synchronous rectifier and turn on the synchronous rectifier when the current crosses zero to a positive value, the synchronous rectifier must be turned off when the current crosses zero to negative value. This is the optimum synchronous rectifier driving mode theoretically. This driving mode is able to avoid the conduction of the body diode of the synchronous rectifier and hence to avoid the additional conduction loss and reverse recovery loss. Two optional current sensing methods including the direct sensing method and indirect sensing method can be employed to sense the current of the synchronous rectifier. The indirect sensing method subtracts the magnetizing current from the transformer primary current to get the reflected secondary current of the synchronous rectifier. The disadvantage of the indirect sensing method is the precision level of the sensed current is not so high. The direct sensing method uses the Hall sensor, current transformer or sensing resistor to directly sense the current of the synchronous rectifier. However, the mentioned method may suffer from the high cost, big size or high loss.
Actually, the voltage driving mode is more popular in most applications. One of the voltage driving modes employs the control signals from the windings (which may be power windings or auxiliary windings) or the circuit node such as the intermediate node of the bridge leg. Another one of the voltage driving modes employs the control signals from the primary MOSFET control signals.
FIG. 1 shows a part of the typical ZVS DC/DC converter circuit. Different connections to the node A gives the different circuit topologies. If two capacitors are connected from the node A to the positive bus and negative bus, this connection gives the asymmetric controlled half bridge topology shown in FIG. 2. If two MOSFETs are connected from the node A to the positive bus and negative bus, this connection gives the phase shifted ZVS full bridge topology shown in FIG. 3.
FIG. 4 shows an asymmetric half bridge topology with the synchronous rectifier applied by the prior art. FIG. 5 shows the key waveform timing diagram applying this prior art. From FIG. 5, the synchronous rectifier S1 is turned on after the current transition of the S1 and S2 and is turned off at tb. Therefore, the conduction time of the body diode has three phases: the current transition phase before the conduction of the synchronous rectifier S1, the primary resonant phase tb˜tc after turning off the synchronous rectifier S1, and the current transition phase tc˜td after turning off the synchronous rectifier S1.
FIG. 6 shows a logic circuit used in phase shift full bridge topology with synchronous rectifiers which was proposed by Vijay Gangadhar Phadke in U.S. Pat. No. 6,504,739. The signals gQ1˜gQ4 are the control signals for the primary MOSFET and the signals gS1˜gS2 are for the secondary MOSFET. FIG. 7 shows the key waveform timing diagram applying this prior art shown in FIG. 6. From FIG. 7, the conduction time of the body diode has two phases: the primary resonant phase ta˜tc after turning off the synchronous rectifier S1, and the current transition phase tc˜td after turning off the synchronous rectifier S1.